*2.7. Performance Testing of Solid Ceramic End Mills*

A cylindrical workpiece with a diameter of 40 mm from a heat-resistant nickel alloy NiCr20TiAl [72] was used as the processed material to carry out comparative performance tests of experimental ceramic end mills and commercial ceramic mills. The chemical composition of the workpiece is shown in Table 4. The hardness (HRC) of the processed material was 34 units, and the ultimate strength is 1150 MPa.

**Table 4.** Chemical composition (wt.%) of processed nickel alloy NiCr20TiAl.


The tests were carried out on a 5-axis turning-milling machining center CTX beta 1250TC (DMG, Hüfingen, Germany). The tool and part clamping system's high rigidity minimizes the risks of accidental brittle fracture of the cutting part of ceramic end mills during resistance tests. A SCHUNK GZB-S Ø20/Ø10 adapter sleeve and an SDF-EC bt40 Ø20 HSK-A63 hydraulic arbor to fix the end mills were used, which allow the tool to be securely clamped with high positioning accuracy.

The tool's tests were carried out according to the program written in the CAD/CAM system GeMMa-3D (LLC STC GeMMa, Zhukovsky, Moscow, Russia) when implementing the strategy of machining the plane of the workpiece end face with an end mill when the cutter moves along a spiral. Figure 7a shows a general view of the ceramic end mill's location relative to the nickel alloy NiCr20TiAl workpiece during testing, and Figure 7b visualizes the processing strategy. The tests were carried out under the following cutting modes: cutting speed *V*c = 376.8 m/min (rotation frequency *n* = 12,000 rpm), feed *S* = 1500 mm/min, and feed per tooth *S*t = 0.031 mm/tooth. The scheme of "dry" processing without cutting fluids was used.

The wear area's critical size *hf* along the end mill tooth's flank face equal to 0.4 mm was taken as the criterion for the loss of performance (failure) of ceramic end mills (Figure 8). The tool's wear resistance was determined as the time of milling until the end mill reached critical wear (when this value was exceeded, the cases of spalling and chipping of the cutting part increased many times). Each end mill sample was researched optically every minute during milling. The performance tests continued until *h*<sup>f</sup> reaches the range of 0.4–0.5 mm to demonstrate the dramatic tendency in tool wear. A metallographic optical microscope of the Stereo Discovery V12 model (Carl Zeiss Vision GmbH, Jena, Germany) was used to quantify wear with a measurement accuracy of ±0.025 mm, which is 5.00–6.25% of the measured value *hf* and less than standard accuracy tolerance. The wear area was monitored on each of the four teeth of the ceramic end mills that were tested. The arithmetic

(**a**) (**b**)

mean values were calculated based on the data obtained, which were used to plot the wear curves of ceramic end mills.

**Figure 7.** Strategy for resistance testing of solid ceramic end mills: (**a**) general view of the ceramic end mill location relative to the NiCr20TiAl nickel alloy workpiece during testing; and (**b**) processing strategy visualization.

**Figure 8.** The wear zone location on the flank face of the ceramic end mill tooth and the *h*<sup>f</sup> area (land) to measure the quantified wear.

A Surftest SJ-410 portable profilometer (Mitutoyo, Kawasaki, Japan) was used for a quantitative and qualitative assessment of a nickel alloy workpiece's surface roughness after machining with various ceramic end mills.
